Titanium alloys are light in weight and high in strength and are used in various fields of parts in which light weight is important, such as aircraft parts and automobile parts. Titanium alloys are superior in corrosion resistance and biocompatibility and are widely used in a field of bioimplant devices. In any of these fields, α-β type titanium alloys, typically exemplified by Ti-6Al-4V, are common since the alloys have high strength and broad utility and are low in cost.
Development of increased strength in α-β type titanium alloys that have high practical utility due to low cost are actively pursued. For example, Japanese Patent Unexamined Publication No. 5-272526 discloses a technique in which Ti-6Al-4V is subjected to gas nitride and a brittle TiN compound surface layer is removed, thereby improving fatigue strength. Japanese Patent Unexamined Publication No. 2000-96208 discloses a technique in which a first layer of a nitrogen solid solution hard layer and a second layer of an oxygen solid solution hard layer are formed simultaneously on pure Ti or Ti-6Al-4V, thereby hardening a surface of the member. Japanese Patent No. 4303821 discloses a composite material in which TiC compound is dispersed in Ti-6Al-4V.
On the other hand, β type titanium alloys also may be exemplified as a high-strength titanium alloy. However, β type titanium alloys include large amounts of rare metals, and materials for forming parts are expensive compared to α-β type titanium alloys. Although static strengths of β type titanium alloys can be improved by aging (precipitating) hardening, fatigue strength is insufficient compared to static strength. Precipitated phases having high hardnesses formed by heat treatment improve static strength. However, difference of hardness (elastic strain) between the precipitated phase and the matrix of the β phase is large, and the boundary between the precipitated phase and β phase may be initiation of breakage in fatigue in which cycle stress is loaded.